Nightguard feels tight around front bottom teeth?

You are not damaging your lower teeth, but I would call your dentist and have it adjusted.  It will take only a few minutes and the soreness that you are experiencing will be gone within a few days.  

A common problem with removable dental appliances  is patient compliance. For that reason reason alone, I would suggest getting your night-guard adjusted. If you're not comfortable, you are less likely to "stick" with it.Best of luck.James Asaf, DDS

Your night guard should be around your top teeth.  This will allow your mandible (bottom jaw)  to move freely. When you have a night guard on the bottom teeth, it is possible for your jaw to get stuck on either side.  Although night guards should fit snug to your teeth, it shouldn't be so tight that your teeth feel sensitive.  I would have it adjusted by your dentist.

Your night guard should fit snuggly around your lower teeth. It will not damage them. If you feel that it is so tight that you won't wear it you might consider asking your dentist to adjust it slightly.